I have had a 10 gal tank, with a single neon tetra and a fat guppy (maybe 2x the size of the neon) for over a year with no problems. Three weeks ago I decided to get more neons, and bought 3 more to add to the tank. The guppy and neons seemed to get along fine. last night when I looked in the tank two of the neons were missing - no bodies or anything, so I'm assuming the guppy ate them or something (I searched the entire tank). Today one of the neons stomach has started to swell (basically it looks like it swallowed a lead ball, perfectly round and deep silver color) its between his fins on the front half of its body. Its coloring is still bright and hes floating fine and eating. Just wondering if this is a disease, pregnancy, or constipation. The other neon looks fine (a bit aggressive to the other, I'm assuming from the stresses of losing two brothers). Also should I isolate him? I want to buy new neons to replace the dead ones, should I wait until the sick one becomes better?

sorry about the confusion. For a year I had one neon and the guppy in a 2gal tank. Three weeks ago, when I upgraded to the 10gal I added 3 more neons. The tank and filters are 3 weeks old. I have changed about 1/3 of the water about a week ago.

Your older fish have been subjected to poor conditions (it is virually impossible to have good conditions in a 2g tank) and so were less shocked than the new fish when ammonia started to rise.
Do 20% water changes twice a week just adding dechlorinator for two gallons of new water each time.
Maybe in about six weeks time your cycle will be finished and it will be safe to drop back water changes to two buckets once a week and maybe add four more neons after having a water sample tested at a fish store with no ammonia or nitrites present.

Thanks for the help... I knew that nitrogen proplems could arise with new tanks, just didn't think it would take 6 weeks. After the first week I thought I was in the clear. Well the best way to learn is by experience... too bad the fish had to pay for my mistakes.

One last thing... any clue what happened to the dead neons... lol there were no bodies.. could the other neons or the guppy have eaton the bodies in less than two days?